The present invention relates in general to devices for recording the position reached by a moving part moved by a rotary motor.
The invention has been developed in particular with a view to the possible use for recording the position reached by one or more axes of an industrial robot.
Presently, in order to record the position reached by such a moving part, it is common to use an angular-position transducer associated with the motor. The transducer, usually consisting of a resolver, generates a signal indicating the angular position reached by the motor during a rotation. The signal generated by the resolver associated with each axis of movement of the robot is transmitted to a general control unit (or servo-control-SC) which keeps a cumulative count of the signal produced by each resolver, thus providing an absolute indication of the position of each moving part of the robot about its own particular axis of movement.
Such a known solution may, however, give rise to considerable drawbacks as regards use, substantially attributable to the fact that, when the robot is separated from its servocontrol (for example, in order to transfer it from the production premises to the user location, or from one user location to another) or when the servocontrol is also momentarily de-energized (for example following a mains power failure), the information relating to the position reached by each moving part is lost.
The drawbacks are particularly great in the case of power supply failures which occur during the operational phases or while the robot is moving: in fact, the robot does not stop immediately when the power supply is interrupted (even if a brake is available to stop the movement). The robot continues to move for a few moments, owing to inertia, until it reaches a final stop position, which is not recorded by the transducer since the latter is no longer energized on account of the loss of power.
The effects of the abovementioned drawbacks may be lessened, at least partly, by using two cascade-connected angular transducers, for example two resolvers coupled together via a speed-reduction mechanism.
According to this configuration, the first resolver registers the momentary position reached, during a rotation, by the motor actuating the moving part, while the second resolver, connected to the first resolver via a speed-reduction mechanism such that the second resolver itself performs a single rotation during the possible travel of the moving part, provides an indication as to where, during the course of this travel, the moving part is located at a particular moment.
This solution, known as master/vernier or coarse/fine type, is impractical both in terms of space (two resolvers and a speed-reduction mechanism must be provided for each axis of movement of the robot) and in terms of cost (two resolvers with the associated precision gearing cost, in fact, more than a single resolver keyed onto the drive shaft).